Examples of the hydraulic systems of the above type include those disclosed, for example, in Japanese Patent Applications Laid-Open No. 6-287952 and Laid-Open No. 2003-130006.
The hydraulic system disclosed in Japanese Patent Application Laid-Open No. 6-287952 relates to a press-in pile driver and is characterized in that an unloading oil path is branched from an oil path between a hydraulic pump as an oil pressure source and a hydraulic cylinder as a load and an on-off valve is inserted in the unloading oil path. The Application Laid-Open No. 6-287952 describes that in this configuration the on-off valve is instantly opened and closed to apply an oil impact to an operating oil flowing from the hydraulic pump to the hydraulic cylinder, whereupon the pressure of the operating oil can be made higher than the discharge pressure from the hydraulic pump. However, the hydraulic system making use of the oil impact causes a considerable loss and has a limit to increase of pressure, and thus it is considered not to be practical.
In contrast to it, the hydraulic system disclosed in the Application Laid-Open No. 2003-130006 is not one making use of the oil impact but is configured to utilize energy storage in an inertial body. As schematically shown in FIG. 1, the hydraulic system 1 described in Laid-Open No. 2003-130006 has a hydraulic pump 4 driven by a driving source 3 having an inertial body 2 like a flywheel, and a load, e.g., a hydraulic motor 6 to which the operating oil discharged from the hydraulic pump 4 is supplied through an oil path 5a. An accumulator 7 is connected to the oil path 5a and a check valve 8 is inserted in the oil path 5a between the accumulator 7 and the hydraulic pump 4. Furthermore, an unloading oil path 5b is branched from the oil path 5a between the check valve 8 and the hydraulic pump 4, and an on-off valve 9 is inserted therein.
In the hydraulic system 1 of this configuration, in a state in which, while the on-off valve 9 is kept open, the hydraulic pump 4 is driven by the driving source 3 and the operating oil is circulated from the oil path 5a via the unloading oil path 5b, the outlet port of the hydraulic pump 4 is in an unloaded condition, and thus the energy outputted from the driving source 3 is stored up as kinetic energy (½)·I·ω2 of the inertial body 2, excluding losses in the hydraulic circuit system, the mechanical system, and so on. Here I is the moment of inertia of the inertial body 2, and ω the angular velocity of the inertial body 2. When the on-off valve 9 is closed at this moment, the operating oil discharged from the hydraulic pump 4 is supplied via the check valve 8 to the load 6; even if the magnitude of the load pressure after the check valve 8 is larger than a pressure of the operating oil that can be discharged from the hydraulic pump 4 driven by a drive torque (Qm) as a potential output of the driving source 3, the operating oil can be supplied at a greater pressure to the load 6 by virtue of a torque resulting from addition of a torque from the kinetic energy of the inertial body 2 preliminarily having stored the energy, to the drive torque (Qm) of the driving source 3, i.e., a torque (Qp) to drive the hydraulic pump 4.
As described above, the hydraulic system 1 disclosed in Laid-Open No. 2003-130006 provides the excellent effect of capability of supplying the operating oil at the higher pressure to the load 6, but also has a problem that usage and application ranges are limited.
For example, there are cases where the distance between the driving source 3 and the on-off valve 9 becomes very long because of an instrument layout. If the range between the driving source 3 and the on-off valve 9 is arranged variable, the oil paths 5a, 5b between the driving source 3 and the on-off valve 9 cannot be constructed of pipes with high rigidity such as steel pipes, but are replaced with rubber hoses or the like in some cases. In such cases, the rubber hoses or the like expand and contract to lower efficiency, and it is difficult to adopt the hydraulic system 1. In addition, in cases where there are a plurality of loads 6 and desires for individual control of the loads, it is necessary to prepare a plurality of hydraulic systems 1 and it is difficult to reduce the number of parts by common use of components.
An object of the present invention is therefore to provide a hydraulic system making use of the energy storage in an inertial body, which can be widely used in a variety of applications and fields.